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Findings Report Promise of Investigational West Nile Vaccine

Apr 17, 2006

Since making an initial appearance in the United States in 1999, more than 19,000 cases of West Nile virus have been reported in this country, resulting in more than 750 fatalities. With the threat of continued epidemics looming, efforts to identify a vaccine for the virus have accelerated. Now, collaborative research between the University of Massachusetts Medical Schools Center for Infectious Disease and Vaccine Research and Acambis Inc., a leading vaccine developer, reports the promise of a recombinant vaccine against West Nile virus.

The paper, A live, attenuated recombinant vaccine against West Nile virus, published in the April 14 early edition online issue of the Proceedings of the National Academy of Sciences, asserts that the vaccine, ChimeriVax-West Nile, rapidly elicited strong immune responses after a single dose in both preclinical tests and in a Phase I human clinical trial. This is the first published report of a West Nile virus vaccine candidate in humans. The paper will appear in the April 25 print edition of the Proceedings of the National Academy of Sciences.

ChimeriVax-West Nile was constructed with Acambis proprietary ChimeriVax technology, which was originally developed in collaboration with St. Louis University. ChimeriVax uses a live, attenuated yellow fever vaccine to create chimeric viruses. In this case, specific genes from the yellow fever virus vaccine that are known to induce immunity to this virus in humans were replaced with the corresponding genes of the West Nile virus. The results of the clinical trial, an 80-subject, randomized double-blind placebo controlled Phase I trial, found that there was no notable difference in the incidence of treatment-emergent systemic reactions between those subjects who received the ChimeriVax-West Nile vaccine and those who received the placebo. Immunogenicity studies revealed that all subjects that received the ChimeriVax-West Nile vaccine developed neutralizing antibodies.

Working with the papers primary author, Thomas P. Monath, MD, chief scientific officer of Acambis, Sharone Green, MD, associate professor of medicine and UMass Medical School colleagues from the Center for Infectious Disease and Vaccine Research including center Director Francis A. Ennis, MD, professor of medicine, and Jeff Kennedy, MD, assistant professor of medicine, led the efforts to determine the cell- mediated immunogenicity, or ability to provoke an immune response from infection fighting white blood cells or T lymphocytes, to the vaccine. The findings demonstrated that the majority of subjects developed West Nile virus-specific T cell responses, responses that play a crucial role in recovery from West Nile virus infection.

While the majority of West Nile infections are mild and will not result in any symptoms, it is believed that 20 percent of those infected will develop mild symptoms such as fever, headache, body aches and swollen glands. According to the Centers for Disease Control and Prevention (CDC), people over the age of 50 are more likely to develop serious symptoms of West Nile virus when infected. West Nile encephalitis, the more severe form of the infection in which the virus crosses the blood-brain barrier and causes inflammation of the brain, is estimated to occur in one out of every 150 of those infected. Symptoms include high fever, neck stiffness, stupor, convulsions, coma and sometimes paralysis. Death can occur in the most severe cases.

There is currently no human vaccine for the virus and efforts to reduce the incidence of infection have revolved around preventive measures to protect against mosquito bites, the cause of nearly all human West Nile virus infections. This research suggests that ChimeriVax-West Nile is a promising candidate that warrants further evaluation to determine its safety and further define its immunogenicity.

Consequently, based on the positive results of this research, Acambis has started the first Phase II clinical trial of a West Nile vaccine, testing ChimeriVax-West Nile in healthy adult subjects. The randomized, double-blind, placebo-controlled Phase II trial is being conducted in more than 200 subjects in the U.S. The safety, tolerability and immunogenicity of Acambis' West Nile virus vaccine will be evaluated at different dosages first in healthy young adults; the optimal dose will then be tested in healthy elderly subjects.